1. Field of the Invention
The present invention relates to an image sensor and particularly to an image sensor suitable for use in facsimile machines, optical character readers and the like. The image sensor of this invention comprises a light emitting section for emitting and irradiating light onto an object, a lens for condensing the light reflected by the object and a light receiving section for receiving the condensed light from the condensing lens.
2. Description of the Related Art
FIG. 1 shows a cross-sectional view of the primary parts of a conventional image sensor, taken along a plane perpendicular to the direction of scan. The image sensor comprises a main frame 50, a transparent covering (glass covering) 51 mounted on the open top of the main frame 50, a linear light emitting section 52 including a base plate 53 which is mounted in the main frame 50, and a light receiving section 54 including a base plate 55 which is mounted on the bottom of the main frame 50. The base plate 53 fixedly supports a resilient piece 57 which resiliently holds an elongated condensing lens (gradient index lens) 56 extending along the light receiving section 54 against the condensing lens mounting wall 50a of the main frame 50. The condensing lens 56 has its optical axis extending in a direction perpendicular to the plane of the transparent covering 51 and the base plate 55. The resilient piece 57 also supports and holds a cylindrical plastic lens 58 directly above the light emitting section 52, the cylindrical lens 58 extending along the length of the light emitting section 52.
In this image sensor, an object to be sensed (e.g. original document) A is brought into contact with the transparent covering 51. The light emitting section 52 emits a diffusing light which is condensed by the plastic lens 58. The condensed light is irradiated onto the original document A slantingly relative to the plane of the transparent covering 51 with an angle equal to about 45 degrees. The light is then reflected by the original document A in a direction perpendicular to the plane of the transparent covering 51. The reflected light passes through the condensing lens 56 and is then received by the light receiving section 54 directly below the lens 56. At the light receiving section 54, the light is converted into an electrical signal.
In the prior art image sensor, loss in the light can be reduced by using the condensing lens to condense the light from the light emitting section. However, the condensing lens is expensive. Since the diameter of the condensing lens can be reduced only to a limited level, it is difficult to reduce the entire system in size. There is a further problem that the distribution of light on the light receiving section has an increased variability. There is a still further problem that the material and assembling cost become expensive since the light emitting and receiving sections are separately disposed within the main frame at different locations. In order to overcome the just mentioned problem, it has been proposed that the light emitting and receiving sections are mounted on the same base plate. However, this proposal raises another problem in that the distance between the original document and the light emitting and/or receiving sections is increased to thus reduce the amount of light.